Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. In certain situations, designers of coated articles may strive for a combination of good visible transmission, low emissivity (or emittance), and blockage of undesirable radiation such as infrared (IR) radiation to prevent or reduce undesirable heating of building or vehicle interiors. High visible transmission for example may permit coated articles to be more desirable in certain window applications, whereas low emissivity (low-E), low SHGC (solar heat gain coefficient), and low SF (solar factor, or g-value) characteristics permit coated articles to block significant amounts of undesirable radiation so as to reduce, for example, undesirable heating of building or vehicle interiors. SF, calculated in accordance with DIN standard 67507 or EN410:1998, relates to a ratio between the total energy entering a room or the like through a glazing and the incident solar energy. Thus, it will be appreciated that low SF values are indicative of good solar protection against undesirable heating of rooms or the like protected by windows/glazings. For example, a low SF value is indicative of a coated article that is capable of keeping a room fairly cool in summertime months during hot ambient conditions. Moreover, the SHGC of an article/window is the fraction of incident solar radiation that is admitted through the article/window (e.g., see NFRC 100-2001 hereby incorporated herein by reference).
Solar control coating systems are known. For example, commonly owned U.S. Pat. No. 5,688,585 discloses a solar control coated article including: glass/Si3N4/NiCr/Si3N4. One object of the '585 patent is to provide a sputter coated layer system that after heat treatment is matchable colorwise with its non-heat treated counterpart. While the coating systems of the '585 patent are excellent for their intended purposes, they suffer from certain disadvantages. In particular, they tend to have rather high emissivity values (e.g., because no silver (Ag) layer is disclosed in the '585 patent).
Low-emissivity (low-E) coating systems are also known in the art. For example, commonly owned U.S. Pat. No. 6,475,626 (hereby incorporated herein by reference) discloses: glass/Si3N4/NiCr/Ag/NiCr/Si3N4. Low-E coating systems of the '626 Patent provide good visible transmission and low-E characteristics. However, coating systems of the '626 Patent cannot achieve a combination of good visible transmission (Tvis) and good solar heat gain coefficient (SHGC). In other words, the coating systems of the '626 Patent have undesirably low Tvis/SHGC ratios. For instance, Example 1 of the '626 Patent in the context of an insulating glass (IG) unit was only able to realize a Tvis/SHGC ratio of about 128. As another example, Example 2 of the '626 Patent in the context of an insulating glass (IG) unit was only able to realize a Tvis/SHGC ratio of about 127, and Example 2 of the '626 Patent monolithically was only able to realize a Tvis/SHGC ratio of about 114.
U.S. Pat. No. 6,782,718 also discloses glass/Si3N4/NiCr/Ag/NiCr/Si3N4. However, the coating systems of the '718 Patent have undesirably low Tvis/SHGC ratios. For instance, the Example in column seventeen of the '718 Patent in the context of an insulating glass (IG) unit was only able to realize a Tvis/SHGC ratio of about 127 (heat treated or HT) or 123 (not HT).
U.S. Pat. No. 5,800,933 discloses another example coated article. However, coated articles of the '933 Patent have undesirably high SHGC values thereby indicating inefficient solar protection against undesirable heating of rooms or the like.
The need for matchability (before heat treatment vs. after heat treatment) is also known with respect to coated articles. Glass substrates are often produced in large quantities and cut to size in order to fulfill the needs of a particular situation such as a new multi-window and door office building, vehicle windshield needs, etc. It is often desirable in such applications that some of the windows and/or doors be heat treated (i.e., tempered, heat strengthened or bent) while others need not be. Office buildings often employ IG units and/or laminates for safety and/or thermal control. It is desirable that the units and/or laminates which are heat treated substantially match their non-heat treated counterparts (e.g., with regard to color, reflectance, and/or the like, at least on the glass side) for architectural and/or aesthetic purposes. U.S. Pat. Nos. 6,014,872 and 5,800,933 (see Example B) disclose a heat treatable low-E layer system including: glassTiO2/Si3N4/NiCr/Ag/NiCr/Si3N4. Unfortunately, when heat treated this low-E layer system is not approximately matchable colorwise with its non-heat treated counterpart (as viewed from the glass side). This is because this low-E layer system has a ΔE* (glass side) value of greater than 4.1 (i.e., for Example B, Δa*G is 1.49, Δb*G is 3.81, and ΔL* (glass side) is not measured; using Equation (1) below then ΔE* on the glass side must necessarily be greater than 4.1 and is probably much higher than that).
Some have attempted to lower SHGC values by providing multiple silver layers in a coating (e.g., low-E coated articles with two silver IR reflecting layers). E.g., see U.S. Pat. No. 7,138,182. However, this is sometimes undesirable in that the coating is more expensive and time consuming to make, and may also suffer from certain durability issues due to the addition of the second silver layer. Thus, it is sometimes desirable to avoid the need for two silver layers in a coating. Moreover, certain coatings with multiple silver layers are difficult to make while maintaining the ability to achieve matchability upon heat treatment (i.e., low ΔE* values).
Others have attempted to lower SHGC values, but this has come at the expense of lower visible transmission values. For example, a currently coating has a stack of: glass/Si3N4(14.3 nm)/NiCr(3.8 nm)/Ag(10.6 nm)/NiCr(2.4 nm)/Si3N4(48.4 nm). While this coated article has a glass side ΔE* value of less than 2 (monolithic) and a SHGC value of 0.35 (monolithic) or 0.30 (IG unit), it can only achieve a visible transmission of 48.4% (monolithic) or 43.4% (IG unit). Accordingly, its Tvis/SHGC ratio is only 138 (monolithic), or 144 (IG, with a low visible transmission of 43.4%). As another example, the double-silver coatings of U.S. Pat. No. 7,138,182 realize a low SHGC, but at the expense of low visible transmission. Thus, the '182 coatings are undesirable in that they both: require two silver layers, and sacrifice visible transmission in order to realize a low SHGC.
In view of the above, it will be apparent to those skilled in the art that there exists a need for a coating or layer system that could satisfy solar control and low-E requirements, visible transmission desires, and ease of manufacturability. In particular, it will be appreciated that there exists a need in the art for a low-E coating, which requires only one silver layer in certain example embodiments, and which can achieve a high visible transmission (Tvis) together with a relatively low solar heat gain coefficient (SHGC) so has to have an improved (i.e., higher) Tvis/SHGC ratio. These features may be provided in the monolithic and/or IG unit contexts.
In certain example embodiments, the coated article if heat treated may also have a low ΔE* value indicating thermal stability upon heat treatment (HT). In other words, there may also exist a need in the art for a low-E coating or layer system which after optional heat treatment substantially matches in color and/or reflection (e.g., as viewed by a naked human eye from the glass side) its non-heat treated counterpart.